Known prior art microelectromechanical (MEM) devices are based on a cantilever beam, as shown in FIG. 1. The beam 10 acts as one plate of a parallel-plate capacitor. A voltage, the actuation voltage, applied between the beam 10 and an electrode 12 on the substrate 14 exerts a force of attraction on the beam 10 which, if the force is large enough, overcomes the stiffness of the beam 10 and causes the beam 10 to bend to contact a secondary electrode 16, thus completing a continuous path. While the prior art MEM device appears to be a simple device, actual implementation meets with a number of drawbacks.
For instance, there tends to be sticking between the beam tip 18 and the secondary electrode 16 so that once closed as a result of the application of the actuation voltage, its removal may not result in the opening of the device. This may occur when the stiction forces overcome the spring restoring forces. In this device, the device opening phase is not electrically, but mechanically controlled, i.e., it is up to "mother nature," embodied in the restoring forces of the beam 10 to effect the opening.
There is also a disadvantageous trade-off between actuation voltage and off isolation. That is, to obtain a low actuation voltage the beam-to-substrate separation should be small, but in turn, a small beam-to-substrate separation results in a large off-parasitic capacitance, thus a low off RF isolation.
Furthermore, the maximum frequency at which the beam can deflect and relax, i.e., turn on/off, is related to its geometry and material properties, in particular, its length, thickness, bulk modulus, and density. Therefore, it may be impossible in some applications to achieve high switching frequencies at practical beam geometries and/or voltages.
One of the intrinsic problems of the cantilever beam device is that the beam's change of state, from open to close, is the result of an instability. Essentially, the beam deforms gradually and predictably, as a function of the applied actuation voltage, up to a threshold. Beyond this threshold, an instability, whereby control is lost, occurs and the beam comes crashing down on the bottom electrode. A number of undesirable conditions result, such as stiction, i.e., the switch remains closed even after removal of the actuation voltage, as well as contact deterioration, which will impair the useful life of the device.